This invention relates to the field of high temperature-oxidation resistant alloys. In particular, this invention relates to MCrAlY powders and a method for forming high-temperature oxidation resistant coatings.
Manufacturers have used composites consisting of aluminum clad nickel (NiAl) and an 80/20 nickel chromium alloy (NiCrAl) for over thirty (30) years as underlayments or bond coats over which other materials may be thermally sprayed.
Industry, for high temperature applications, utilizes a particular thermal spray system consisting of a metallic underlayment overcoated with a ceramic layer. This coating system is referred to as a xe2x80x9cThermal Barrier Coatingxe2x80x9d or TBC.
TBCs are thermal insulators that shield the host or substrate material from the hot gases flowing over them. The metallic underlayment or bond coat is a derivative family of alloys described as xe2x80x9cMCrAlYsxe2x80x9d. MCrAlYs are useful because they exhibit excellent resistance to oxidation and hot corrosion. These alloys, where the xe2x80x9cMxe2x80x9d represents a metal, may be either iron (Fe), nickel (Ni) or cobalt (Co) or alloys thereof such as iron-base alloys, nickel-base alloys and cobalt-base alloys. They are generically referred to as FeCrAlYs, NiCrAlYs or CoCrAlYs.
When thermally sprayed, as with a plasma or combustion gas apparatus, MCrAlY coatings attach to the host substrate by mechanical means. These coatings form from powders locked or keyed onto asperities previously fashioned onto the substrate surfacexe2x80x94they do not metallurgically or chemically attach to the substrate. Manufacturers sometimes rely on a lenghty post-spray, high temperature diffusion cycle to promote adhesive strength. This diffusion treatment provides base metal substrates shielding during environmental exposure. Without diffusion, corrosive media can penetrate the coating.
For example, U.S. Pat. Nos. 3,322,515 and 3,436,246 disclose Ni5Al/NiCr5Al composites that promote coating attachment. These composite materials form metallurgically bonded, or xe2x80x9cself-bondedxe2x80x9d materials. When exiting a thermal spray device, both materials react exothermically to produce a xe2x80x9csuperheatedxe2x80x9d liquid that metallurgically bonds upon impact with a metallic substrate. Unfortunately, both materials lack the long term oxidation and hot corrosion resistance necessary for applications at temperatures above 650xc2x0 C. (1200xc2x0 F.).
As noted above, MCrAlYs, by their nature, serve as excellent xe2x80x9cbarrier/bondxe2x80x9d materials in thermal barrier coating (TBC) applications. Commercial applications of these TBCs include use in diesel and gas turbine engines. As xe2x80x9cbarrier/bondxe2x80x9d coats, MCrAlYs provide an xe2x80x9canchoragexe2x80x9d, or xe2x80x9cbondxe2x80x9d, for subsequent depositsxe2x80x94usually a ceramic coating.
Additionally, the aluminum and chromium constituents form an oxide film or xe2x80x9cbarrierxe2x80x9d that protects the less noble substrate from the hot-corrosive operating environment. If the gases penetrate through to the coating/substrate interface, degradation of the less noble substrate could occur and eventually cause spallation and detachment of the coating. Therefore, a tightly adherent MCrAlY coating would extend a coating""s useful service life.
Because of the protective alumina and chromia films that MCrAlYs form, they may be also be used after a diffusion cycle as stand alone coatings. These stand alone coatings provide resistance to both oxidation and hot corrosion attack.
MCrAlY coatings are also useful as xe2x80x9crub tolerantxe2x80x9d deposits in gas path seal (dimension or clearance control) systems. Rub-tolerant MCrAlY coatings are used on static components (seal segments) in the turbine section of aero gas turbine engines. During engine operation the rotating, hot blades lengthen and graze the coating. This incursion causes both the MCrAlY and blade tips to smear. The coating is gouged and blade tips worn, i.e. the blade loses some of its original length. Loss of material from either component increases the distance between them. Increased distances translate to increased airflow and an ultimate loss in engine efficiency. If the MCrAlY seals were abradable rather than rub tolerant, the blade would cut away the abradable material without damaging the blade. Furthermore, the abradable material would maintain tighter tolerances between the blade tips and seal segments. Not only would this enhance engine performance; it would decrease the need for either blade repair or replacement.
For many years, these abradable properties and functions have been sought by the aero and industrial gas turbine industries. For example, U.S. Pat. No. 3,084,064 relies on a mechanical mixture of a metallic component with a softer material, such as, carbon or graphite. The softer component forms a void that contributes to the coatings"" abradability. Manufacturers must apply MCrAlYs as dense coatings, however, to reduce corrosive gas infiltration. Furthermore, parametric manipulation to reduce MCrAlY coating density and enhance abradability, results in loose deposits that exhibit both poor adhesion and inadequate cohesion.
It is object of this invention to provide an MCrAlY coating that tightly bonds to its substrate without enduring a high temperature diffusion cycle.
It is a further object of this invention to provide an MCrAlY coating that may serve as a TBC barrier/bond or a stand-alone high temperature oxidation hot corrosion resistant layer.
It is a further object of this invention to provide an abradable MCrAlY coating for clearance control (seal) applications.
A material useful for forming high temperature coatings The material contains a MCrAlY powder wherein M is selected from the group consisting of iron, nickel, cobalt, iron-base alloys, nickel-base alloys and cobalt-base alloys. An aluminum layer coats the MCrAlY powder.
The method forms a high temperature coating with a MCrAlY powder. The M is selected from the group consisting of iron, nickel, cobalt, iron-base alloys, nickel-base alloys and cobalt-base alloys. An aluminum coating covers each particle of the MCrAlY powder. Thermally spraying the MCrAlY powder towards a substrate reacts the aluminum coating with the MCrAlY powder to metallurgically bond the MCrAlY powder and coat the substrate.